1. Field of the Invention
The present invention relates to modifying the pulse of coherent energy used in shock processing of solid materials, and more particularly to methods and apparatus for improving the functionality, quality, and usefulness of a pulse of coherent energy in laser shock processing.
2. Description of the Related Art
Laser shock processing involves directing a pulse of coherent radiation to a piece of solid material to produce shock waves therein. The produced shock wave causes compressive residual stresses to form within the solid material. These compressive residual stresses improve the fatigue hardness and corrosion resistance properties of the solid material.
Laser shock processing utilizes a laser comprised of an oscillator, amplifiers, lenses, and irises. Depending on the type of oscillator used and the aperture size of the oscillator, the laser beam is either of a single transverse mode or a multi-transverse mode.
One problem with current lasers used in laser shock processing is potential damage that may occur to components downstream of a hard iris. Traditionally, the iris used in a laser is composed of a hard material, such as aluminum. When a laser beam exceeds the diameter of a hard iris, the intensity of the beam downstream is modulated and diffraction fringes are formed. The fringing is produced by diffracted coherent light interfering with the main beam. The diffraction fringes create hot spots, or areas of higher energy. These hot spots may lead to optical damage in components downstream or upstream. For example, amplified diffraction fringes may lead to damage to the laser gain medium or to laser optical components and their coatings.
An additional problem with current hard irises is that there is an increase in divergence of the beam as the beam passes through the hard iris. The divergence of the beam alters how the beam propagates, which in turn, produces a less uniform spacial distribution of beam output. The resulting beam output from a laser utilizing a hard iris is less effective in laser shock processing as compared to a more uniform spacial energy distribution of a non-diffracted beam output.
Another problem with current lasers used in laser shock processing is the creation of hot zones. This is especially a problem in multi-transverse mode laser oscillators. In multi-transverse mode oscillator lasers, areas of higher amplitude or hot zones, naturally occur on the outside edges of a cross-section of a laser beam. Amplification of a laser beam with hot zones further increases these hot zones which may result in possible damage to the optical components of the laser.